ThinkPHP框架，mysql数据库主从，读写分离源码分析

贴出相关代码段

[文件] 数据库中间层实现类 ThinkPHP/Lib/Core/Db.class.php
```php

class Db {
    ......
    /**
     * 初始化数据库连接
     * @access protected
     * @param boolean $master 主服务器
     * @return void
     */
    protected function initConnect($master=true) {
        if(1 == C('DB_DEPLOY_TYPE'))
            // 采用分布式数据库
            $this->_linkID = $this->multiConnect($master);
        else
            // 默认单数据库
            if ( !$this->connected ) $this->_linkID = $this->connect();
    }

/**
     * 连接分布式服务器
     * @access protected
     * @param boolean $master 主服务器
     * @return void
     */
    protected function multiConnect($master=false) {
        static $_config = array();
        if(empty($_config)) {
            // 缓存分布式数据库配置解析
            foreach ($this->config as $key=>$val){
                $_config[$key]      =   explode(',',$val);
            }
        }
        // 数据库读写是否分离
        if(C('DB_RW_SEPARATE')){
            // 主从式采用读写分离
            if($master)
                // 主服务器写入
                $r  =   floor(mt_rand(0,C('DB_MASTER_NUM')-1));
            else{
                if(is_numeric(C('DB_SLAVE_NO'))) {// 指定服务器读
                    $r = C('DB_SLAVE_NO');
                }else{
                    // 读操作连接从服务器
                    $r = floor(mt_rand(C('DB_MASTER_NUM'),count($_config['hostname'])-1));   // 每次随机连接的数据库
                }
            }
        }else{
            // 读写操作不区分服务器
            $r = floor(mt_rand(0,count($_config['hostname'])-1));   // 每次随机连接的数据库
        }
        $db_config = array(
            'username'  =>  isset($_config['username'][$r])?$_config['username'][$r]:$_config['username'][0],
            'password'  =>  isset($_config['password'][$r])?$_config['password'][$r]:$_config['password'][0],
            'hostname'  =>  isset($_config['hostname'][$r])?$_config['hostname'][$r]:$_config['hostname'][0],
            'hostport'  =>  isset($_config['hostport'][$r])?$_config['hostport'][$r]:$_config['hostport'][0],
            'database'  =>  isset($_config['database'][$r])?$_config['database'][$r]:$_config['database'][0],
            'dsn'       =>  isset($_config['dsn'][$r])?$_config['dsn'][$r]:$_config['dsn'][0],
            'params'    =>  isset($_config['params'][$r])?$_config['params'][$r]:$_config['params'][0],
        );
        return $this->connect($db_config,$r);
    }
```

[文件] PDO数据库驱动  ThinkPHP/Extend/Driver/Db/DbPdo.class.php

```php
class DbPdo extends Db{
    ......
    /**
     * 执行查询 返回数据集
     * @access public
     * @param string $str  sql指令
     * @return mixed
     */
    public function query($str) {
        $this->initConnect(false);
        if ( !$this->_linkID ) return false;
        $this->queryStr = $str;
        //释放前次的查询结果
        if ( !empty($this->PDOStatement) ) $this->free();
        N('db_query',1);
        // 记录开始执行时间
        G('queryStartTime');
        $this->PDOStatement = $this->_linkID->prepare($str);
        if(false === $this->PDOStatement)
            throw_exception($this->error());
        $result =   $this->PDOStatement->execute();
        $this->debug();
        if ( false === $result ) {
            $this->error();
            return false;
        } else {
            return $this->getAll();
        }
    }

/**
     * 执行语句
     * @access public
     * @param string $str  sql指令
     * @return integer
     */
    public function execute($str) {
        $this->initConnect(true);
        if ( !$this->_linkID ) return false;
        $this->queryStr = $str;
        $flag = false;
        if($this->dbType == 'OCI')
        {
            if(preg_match("/^\s*(INSERT\s+INTO)\s+(\w+)\s+/i", $this->queryStr, $match)) {
                $this->table = C("DB_SEQUENCE_PREFIX").str_ireplace(C("DB_PREFIX"), "", $match[2]);
                $flag = (boolean)$this->query("SELECT * FROM user_sequences WHERE sequence_name='" . strtoupper($this->table) . "'");
            }
        }//modify by wyfeng at 2009.08.28
        //释放前次的查询结果
        if ( !empty($this->PDOStatement) ) $this->free();
        N('db_write',1);
        // 记录开始执行时间
        G('queryStartTime');
        $this->PDOStatement    =    $this->_linkID->prepare($str);
        if(false === $this->PDOStatement) {
            throw_exception($this->error());
        }
        $result    =    $this->PDOStatement->execute();
        $this->debug();
        if ( false === $result) {
            $this->error();
            return false;
        } else {
            $this->numRows = $this->PDOStatement->rowCount();
            if($flag || preg_match("/^\s*(INSERT\s+INTO|REPLACE\s+INTO)\s+/i", $str)) {
                $this->lastInsID = $this->getLastInsertId();
            }
            return $this->numRows;
        }
    } 
```